Human reproduction is
sexual reproduction that results in
human fertilization to produce a human offspring. It typically involves
sexual intercourse
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...
between a
sexually mature human
male and
female.
During sexual intercourse, the interaction between the
male and
female reproductive systems results in
fertilization of the
ovum
The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...
by the
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
to form a zygote.
These specialized reproductive
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
are called
gametes, which are created in a process called
gametogenesis.
While normal cells contain 46
chromosomes (23 pairs), gamete cells only contain 23 single chromosomes, and it is when these two cells merge into one
zygote cell that
genetic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryo ...
occurs and the new zygote contains 23 chromosomes from each parent, giving it 46 chromosomes (23 pairs).
The zygote then undergoes a defined development process that is known as human embryogenesis, and this starts the typical 9-month
gestation period
In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it ...
that is followed by
childbirth.
The fertilization of the ovum may be achieved by
artificial insemination methods, which do not involve sexual intercourse.
Assisted reproductive technology
Assisted reproductive technology (ART) includes medical procedures used primarily to address infertility. This subject involves procedures such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), cryopreservation of gametes o ...
also exists.
Biological and legal requirements
In order for human reproduction to be achieved, an individual must have undergone
puberty first, requiring
ovulation in females and the
spermarche in males to have occurred prior to engaging in
sexual intercourse
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...
or achieving
pregnancy through non-penetrative means. Before puberty, humans are
infertile
Infertility is the inability of a person, animal or plant to reproduce by natural means. It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). It is the normal state ...
, as their genitals lack reproductive function (only being able to discharge
urine).
Legal factors also play a vital role in the achievement of human reproduction: a
minor
Minor may refer to:
* Minor (law), a person under the age of certain legal activities.
** A person who has not reached the age of majority
* Academic minor, a secondary field of study in undergraduate education
Music theory
*Minor chord
** Barb ...
under the
age of consent
The age of consent is the age at which a person is considered to be legally competent to consent to sexual acts. Consequently, an adult who engages in sexual activity with a person younger than the age of consent is unable to legally claim ...
cannot give legal consent to sexual intercourse or artificial alternatives to reproduction, the former case of which is liable to have the older party charged with
statutory rape, depending on jurisdictions.
Even for minors above the age of consent,
comprehensive sex education advises both consenting parties to use
contraception to avoid both
sexually transmitted diseases and
early, unplanned/unwanted pregnancies. Pregnancy in girls under the age of 15 is especially discouraged due to their reproductive systems having yet to reach full maturity.
Anatomy
Male reproductive system
The male reproductive system contains two main divisions: the
testes where
sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
are produced, and the
penis which discharges the sperm as
semen (this is known as an
ejaculation). In humans, both of these organs are outside the
abdominal cavity
The abdominal cavity is a large body cavity in humans and many other animals that contains many organs. It is a part of the abdominopelvic cavity. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the th ...
. Having the testes outside the abdomen facilitates temperature regulation of the sperm, which require specific temperatures to survive about 2-3 °C less than the normal body temperature i.e. 37 °C. In particular, the extraperitoneal location of the testes may result in a 2-fold reduction in the heat-induced contribution to the spontaneous mutation rate in male germinal tissues compared to tissues at 37 °C. If the testicles remain too close to the body, it is likely that the increase in temperature will harm the spermatozoa formation, making conception more difficult. This is why the testes are carried in an external
scrotum rather than within the abdomen; they normally remain slightly cooler than body temperature, facilitating sperm production.
Female reproductive system
The female reproductive system likewise contains two main divisions: the external genitalia and the internal genitalia.
The ovum meets with the
sperm cell
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
: a sperm may penetrate and merge with the egg, fertilizing it with the help of certain hydrolytic enzymes present in the acrosome. The fertilization usually occurs in the
oviducts, but can happen in the uterus itself. The
zygote then becomes
implanted in the lining of the uterus, where it begins the processes of
embryogenesis
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
and
morphogenesis. When the
fetus is developed enough to survive outside of the
uterus, the cervix dilates and contractions of the uterus propel it through the birth canal, which is the vagina, and thereby gives external life to the newborn
infant
An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
. This process is called
childbirth.
The ova, which are the female sex cells, are much larger than the spermatozoon and are normally formed within the ovaries of the female fetus before birth. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the later
zygote and
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
. Over a regular interval known as the
menstrual cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
, in response to hormonal signals, a process of
oogenesis
Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.
O ...
matures one ovum which is released and sent down the Fallopian tube. If not fertilized, this egg is flushed out of the system through
menstruation
Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of hor ...
.
Process of fertilization
Human reproduction normally begins with copulation, though it may be achieved through
artificial insemination, and is followed by nine months of pregnancy before childbirth. Pregnancy can be avoided with the use of contraceptives such as
condom
A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both male and female condoms. With proper use—and use at every act of in ...
s and
intrauterine device
An intrauterine device (IUD), also known as intrauterine contraceptive device (IUCD or ICD) or coil, is a small, often T-shaped birth control device that is inserted into the uterus to prevent pregnancy. IUDs are one form of long-acting rever ...
s.
Copulation
Human reproduction naturally takes place as
internal fertilization by sexual intercourse. During this process, the man inserts his erect
penis into the woman's
vagina and then either partner initiates rhythmic
pelvic thrusts until the male
ejaculates semen, which contains sperm, into the vaginal canal. The sperm and the ovum are known as the
gametes (each containing half the genetic information of the parent, created through
meiosis).
The sperm (being one of approximately 250 million sperm in a typical ejaculation) travels through the vagina and cervix into the uterus or Fallopian tubes. Only 1 in 14 million of the ejaculated sperm will reach the Fallopian tube. The egg simultaneously moves through the Fallopian tube away from the ovary. One of the sperm encounters, penetrates and fertilizes the ovum, creating a
zygote. Upon
fertilization and
implantation, gestation of the fetus then occurs within the uterus.
Pregnancy rates for sexual intercourse are highest during the
menstrual cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
time from some 5 days before until 1 to 2 days after ovulation. For optimal pregnancy chance, there are recommendations of sexual intercourse every 1 or 2 days, or every 2 or 3 days. Studies have shown no significant difference between different
sex positions and pregnancy rate, as long as it results in
ejaculation into the
vagina.
Alternative methods
As an alternative to natural sexual intercourse, there exists
artificial insemination, where sperm is introduced into the female reproductive system without the insertion of the penis.
There are also many methods of
assisted reproductive technology
Assisted reproductive technology (ART) includes medical procedures used primarily to address infertility. This subject involves procedures such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), cryopreservation of gametes o ...
, such as
in vitro fertilization, where one or more
egg cells are retrieved from a woman's ovaries and co-incubated with sperm outside the body. The resulting embryo can then be reinserted into the womb of the woman.
Pregnancy
Pregnancy is the period of time during which the fetus develops, dividing via
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
inside the uterus. During this time, the fetus receives all of its
nutrition and oxygenated blood from the mother, filtered through the
placenta, which is attached to the fetus'
abdomenvia an
umbilical cord. This drain of
nutrients can be quite taxing on the mother, who is required to ingest slightly higher levels of
calories. In addition, certain
vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. Gestation period is about 266 days in humans. While in the uterus, the baby first endures a very brief zygote stage, then the embryonic stage, which is marked by the development of major organs and lasts for approximately eight weeks, then the fetal stage, which revolves around the development of bone cells while the fetus continues to grow in size. It is estimated that about 3-5% of couples are infertile and the fecunditity of couples is around 30% for each menstrual cycle.
Labor and birth
Labor is separated into 4 stages. The first stage involves latent phase and active phase separated by the dilation of the cervix for 6 to 10 cm. The second stage is the pushing stage. The third stage involves the delivery of the placenta. And the last stage is the contraction of the uterus.
Once the fetus is sufficiently developed, chemical signals begin the process of birth, which begins with the fetus being pushed out of the birthing canal. The newborn, which is called an
infant
An infant or baby is the very young offspring of human beings. ''Infant'' (from the Latin word ''infans'', meaning 'unable to speak' or 'speechless') is a formal or specialised synonym for the common term ''baby''. The terms may also be used to ...
in humans, should typically begin
respiration on its own shortly after birth. Not long after, the placenta eventually falls off on its own. The person assisting the birth may also sever the umbilical cord.
Discovery of mechanism
While most ancient human societies believed that
sexual intercourse
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...
was necessary for reproduction, the reasons some sex did not result in children, and the mechanism by which mating produced children were not understood. The theory of
preformationism was popular in
Ancient Greece and
Christendom for centuries. Because they are too small to see with the naked eye, it was only after his invention of the microscope that
Antonie van Leeuwenhoek
Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...
discovered
spermatozoa in 1677.
Mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
and
meiosis were not discovered until the late 1800s.
See also
*
Heterosexuality
Heterosexuality is romantic attraction, sexual attraction or sexual behavior between people of the opposite sex or gender. As a sexual orientation, heterosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to ...
*
Antinatalism
*
Evolution of sexual reproduction
*
Female infertility
*''
Human Reproduction (journal)''
*''
Journal of Human Reproductive Sciences''
*
Male infertility
Male infertility refers to a sexually mature male's inability to impregnate a fertile female. In humans it accounts for 40–50% of infertility. It affects approximately 7% of all men. Male infertility is commonly due to deficiencies in the semen, ...
*
Natalism
Natalism (also called pronatalism or the pro-birth position) is an ideology that promotes the reproduction of human life as the preeminent objective of being human.
Compare:
The term, as it relates to the belief itself, comes from the French wor ...
*
Paternal age effect
*
Reproduction
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
*
Reproductive system
*
Legislation on human reproduction
*
Reproductive health
References
Altrui - The Human Egg Cell Explained
Further reading
* Hopwood, Nick, Rebecca Flemming, Lauren Kassell, eds. ''Reproduction: Antiquity to the Present Day'' (Cambridge UP, 2018). Illustrations. xxxv + 730 pp.
excerptals
online review44 scholarly essays by historians.
*
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